The present invention relates to vibratory-energy-beam imaging and more particularly, to a novel method and apparatus for increasing the resolution and/or signal-to-noise ratio of a vibratory energy beam, and particularly a medical ultrasonic imaging system beam, by utilization of dynamic focusing and coded excitation techniques during beam transmission.
Beamed vibratory energy imaging systems, such as medical ultrasound imaging systems and the like, routinely enhance the spatial resolution of pulse-echo images by dynamically focusing the signals received by a phased array transducer into a narrow beam. The time-dependent delay provided in each receiver channel phases the receive signals into a beam focused at some range R, and some angle .theta., with respect to the transducer array normal. However, the ultimate resolution of pulse-echo images is determined not by the receiver beam pattern alone, but rather by the product of the receiver beam pattern with the transmitter beam pattern. Thus, dynamic reception focusing, while greatly increasing pulse-echo image resolution (especially in the near field), leaves much to be desired with resolution at greater field distances. During transmission, since the array transducers have hitherto been excited with an impulse driving function on transmit, the conventional system transmit beam has only been focused (by a mechanical lens configuration or a fixed time delay sequence) at a single range, i.e. in a single focal zone. Consequently, outside this transmitter fixed focal zone, the resolution of the image is severely limited by the characteristics of the transmitted beam, even though the imaging system resolution was very good within that transmitter focal zone. Thus, even though the receiver provides an in-focus beam in regions outside of the transmitter focal zone, the imaging system resolution is still degraded. Therefore, to obtain high resolution in the entire image plane, it is desirable to increase resolution by dynamically-focusing the transmission beam so that both the transmit and receive beams are focused at the various ranges along each radial of a scanned sector. Because each transmitted beam, even though focused in an associated zone, provides some excitation of other zones of a sample-to-be-imaged, some means must be provided to reject responses from outside that focal zone.